1. Field of the Invention
The present invention relates to electrostatic copying apparatus in general.
More particularly, the invention relates to improvements in electrostatic copying apparatus wherein a mixture of toner particles and of carrier particles is cascaded over a moving charge-carrier (xerographic) surface, in order to convert electrostatic latent images on this surface into powder images.
Still more specifically, the invention relates to improvements in electrostatic copying apparatus wherein the electrostatic charging of toner-carrier mix occurs as the result of triboelectric effects during circulation of the mix at the developing station.
2. The Prior Art
In the type of electrostatic copying apparatus under discussion, the mixture of toner particles and carrier particles is charged as a result of triboelectric interaction between the particles and the particle-contacting surfaces of apparatus components at the developing station. This results in widely differing interactions which, as a rule, cause the particles of the toner-carrier mix to accumulate a charge that differs from the charge of the overall body of mixture. To insure contrasting development of a latent image, i.e. the transfer of a sufficient quantity of toner particles to the image areas and the absence of such particles on the non-image areas of the charge-carrier surface, the charges must be so controlled that the ratio of the charge constituting the latent image to the charge of the toner particles and of the carrier particles will remain within a very narrow range. As a general rule, the toner particles tend to adhere to non-image areas of the charge-carrier surface, due to the presence of such adhesive forces as e.g. residual charge on non-image areas of the surface.
A proposal for an apparatus offering such control-possibilities has been made in U.S. Pat. No. 3,863,602, granted to Joachim Mueller et al. This apparatus has highly conductive particle-contacting surfaces whose purpose it is to improve the homogeneity (uniformity) of the triboelectric charges imparted to the particles of the toner-carrier mixture. It produces a slight discharge of the triboelectric charges of the particles in the toner-carrier particle mixture, to the point where the overall charge of the mixture is substantially uniform and at or near ground potential. As a result, the mixture of carrier and toner particles has been found to produce very good powder images on the xerographic surfaces, even of their lines and of low-contrast areas. However, the elimination of the outwardly acting residual charges of the mixture, resulting from the triboelectric charging and having a polarity opposite to that of the toner charge, necessarily increases the tendency of the apparatus to form undesired images of background areas having a residual charge potential on the xerographic surface. The patent proposes to counteract this by preventing or interrupting a total discharge of the toner-carrier mixture, in that the mixture is made to contact--during part of its travel path--surfaces which exhibit neutral characteristics with respect to the overall triboelectric charge of the mixture. This, however, permits only a relatively casual selection of the desired charge condition since precision in the selection is not possible.